As used herein, the term "die drool" means the accumulation of polymeric or mostly polymeric material at the external die surface adjacent to the polymer exiting the die. Such build-up causes the surface of the polymer composition exiting the die to have increased roughness. Portions of the material build-up may re-attach itself to the extrudate causing upsets or poor product. The phenomenon is also described in the publication entitled, "Die Drool--What Causes It; How to Avoid It" by Imrich Klein from "Plastics World"--May, 1981.
The term "filled polymeric resin" means any solid additives which are incorporated in the resin prior to extrusion. These can include carbon black, calcium carbonate, silica, hydrated aluminum and other additives normally added in the production of jacketing insulation for wire and cable products.
The term "highly filled polymeric resin" differentiates polymer systems of low volumetric loading from those of high volumetric loading. In particular, high volumetric loading of fillers are associated with a marked decrease in extensibility of the molten polymer system. Extensibility is the strain at break under a defined rate of extension and temperature. The strain can be estimated in an engineering measure as L/L.sub.0, where L is the length at break and L.sub.0 is the initial length of the deforming section. In general, the volumetric loading of a highly filled polymer system is greater than 10%, preferably greater than 15% to as high as 40% for most thermoplastic systems. The purpose of working at such high levels is to incorporate the properties of the filler into the polymeric system. One such purpose is to obtain flame retardancy not normally found in the polymer resin. Another reason may be to increase the modulus or toughness of the polymer resin. An economic reason may be to simply decrease cost at little or no loss in properties.
The use of the measure, "a marked decrease in extensibility of the molten polymer system" can be used as a relevant working definition of high loadings. This allows one to separate the specific mechanism of die drool associated with high filler loadings independent of the type of filler used. A simple test is to stretch the filled molten polymer at rates and at temperatures corresponding to the particular extrusion operation of concern. If the strain at break is less than about 3 or 4, then the material is relatively brittle compared to lower filler loadings which would yield a ductile response and not fail at strains less than about 10.